ends of the nine-ring tail fibrils (Fig. 1). The central tail fibrils extend on through the distal centriole into the proximal centriole. The neckpiece contains the greatest concentration of mitochondrial granules found in the spermatozoan and, in addition, remnant (Figs. (Fig. 2) a septal structure which may be the golgi 2 and 3). The tail sheath is a double membranous structure surrounding the tail fibrils. cross section (Figs. 4, 5, It is spindle-shaped in and 6) had been assumed previously. rather than round as The sheath is semipermeable as evidenced by marked swelling with fixation in hypotonic solution. Mitochondrial granules are found throughout the length of the tail between the sheath and the axial filaments (Fig. 4). The larger granules are composed of subunits and are never found with the ring formed by the ring fibrils. In the tail fibrils evidence of substructure within each fibril is found in cross sections of the tails 4, 5 and 6). (Figs. There is no lateral association between adjacent fibrils, the diameters of which are not altered with changes in osmotic pressure.